Hydrates are formed of two components, water and certain gas molecules, for example, alkanes having 1–4 carbon atoms, such as those found in natural gas or petroleum gas, for example, methane, ethane, propane, n-butane, isobutane, H2S and/or CO2. These ‘gas’ hydrates will form under certain conditions, i.e. when the water is in the presence of the gas and when the conditions of high pressure and low temperature reach respective threshold values. The gas may be in the free state or dissolved in a liquid phase such as a liquid hydrocarbon.
The formation of such hydrates can cause problems in the oil and gas industries.
The problem is particularly of concern as natural gas and gas condensate resources are discovered where operating conditions surpass these threshold values, i.e. in deep cold water and on-shore in colder climates.
The problem, of hydrate formation may occur during gas transportation and processing, the solid hydrate precipitating from moist gas mixtures. This is particularly true with natural gas which when extracted from the well is normally saturated with water. Often in such a case, at cold temperatures (for example, temperatures of less than 10° C.), hydrates will form in downstream transportation networks and this can cause large pressure drops throughout the system and reduce or stop the flow of natural gas.
A typical situation where gas hydrate formation can occur is in off shore operations. When produced fluids comprising gas and water reach the surface of the seabed, the lowering of the temperature of the produced fluids (through heat exchange with the sea water which is typically at a temperature of 3 to 4° C. at the sea bed) generally results in the thermodynamic conditions for hydrates to form. Thus, as the fluids are transported either in a long vertical pipeline, for example, a riser system or through a pipeline laid along the seabed, solid gas hydrates may block the riser system or pipeline.
Several methods are known to prevent hydrate formation and subsequent problems in pipelines, valves and other processing equipment.
Physical methods have been used, e.g. insulation of pipelines in such a way as to avoid the transported produced fluids being cooled to below the threshold value for formation of hydrates under the operating pressure of the pipeline; drying the fluid before introduction into the pipeline; or lowering the pressure in the system. However, these techniques are either expensive or are undesirable because of loss of efficiency and production.
Chemical procedures have also been used. Electrolytes, for example, ammonia, aqueous sodium chloride, brines and aqueous sugar solutions may be added to the system.
Alternatively, the addition of methanol or other polar organic substances, for example, ethylene glycol or other glycols may be employed. Although methanol injection has been used widely to inhibit hydrate formation, it is only effective if a sufficiently high concentration (for example, 10 to 50% by weight of the water content) is present since at low concentrations there is the problem of facilitation of hydrate formation. Also, for methanol to be used economically under cold environmental conditions there must be early separation and expulsion of free water from the well in order to minimise methanol losses in the water phase.
According to U.S. Pat. No. 4,856,593, stoppage of gas production from gas wells may be prevented by incorporating in the gas a surface active agent which inhibits the formation of gas hydrates and/or the agglomeration of hydrate crystallites into large crystalline masses which are capable of blocking gas flow. The surface active agent may be introduced into a gas well through a workstring and co-mingles with natural gas flowing from the subterranean formation. Examples of surface active agents which may be employed include such materials as phosphonates, phosphate esters, phosphonic acids, esters of phosphonic acids, inorganic polyphosphates, salts and esters of inorganic polyphosphates, and polymers such as polyacrylamides and polyacrylates.
The use of certain amphiphilic compounds to lower the hydrate formation temperature and/or to modify the mechanism of formation of such hydrates is described in U.S. Pat. No. 4,915,176. The amphiphilic compounds may be non ionic, anionic or cationic. Examples of non ionic amphiphilic compounds include the oxyethylated fatty alcohols, the alcoxylated alkylphenols, the oxyethylated and/or oxypropylated derivatives, sugar ethers, polyol esters, such as glycerol, polyethylene glycol, sorbitol or sorbitan, sugar esters, niono and diethanolamides, carboxylic acid amides, sulfonic acids or amino acids. Suitable anionic amphiphilic compounds include carboxylates, such as metal soaps, alkaline soaps or organic soaps (such as N-acylaminoacids, N-acylsarcosinates, N-acylglutamates, N-acylpolypeptides); sulfonates such as alkylbenzenesulfonates or sulfosuccinic derivatives; sulfates such as alkylsulfates, alkylethersulfates, and phosphates. Among the cationic amphiphilic compounds are alkylamine salts, quaternary ammonium salts, such as alkyltrimethylammonium derivatives, alkyltriethylammonium derivatives, alkyldimethylbenzylammonium derivatives, alcoxylated alkylamine derivatives, heterocyclic derivatives, such as pyridinium, imidazolinium, quinolinium, piperidinium or morpholinium derivatives.
U.S. Pat. No. 4,973,775 describes the use of amphiphilic compounds, in particular, non-ionic amphiphilic compounds or amphiphilic compounds including an amide group to delay the formation and/or reduce the agglomeration tendency of hydrates in conditions where a hydrate may be formed. The amide compounds may be hydroxylated amide compounds, notably carbylamides of substituted or unsubstituted carboxylic acids, carbylamides of amino acids such as peptides, or sulfonic acid amides.
U.S. Pat. No. 5,877,361 describes a process which allows a hydrate-dispersing additive to be at least partly recovered and recycled. The method is said to be particularly advantageous when the amount of liquid hydrocarbon phase, oil or condensate is such that a water-in-oil emulsion may form. It is said to be possible to use the recovery technique during the production of condensate gas or of oil with associated gas since, in both cases, the presence of a liquid hydrocarbon phase is certain in the production pipe, from the wellhead to the separator or to the terminal. The hydrate-dispersing additive fed into the liquid hydrocarbon phase disperses the water and the hydrates after the formation thereof within the liquid hydrocarbon phase, thus ensuring their transportation in the dispersed form. Suitable dispersing additives are polyol and carboxylic acid esters or carboxylic acid hydroxycarbylamides.